Pressure sensors can be constructed using a number of technologies. One technology that has been used extensively is strain gauge technology. While strain gauges provide reproducible results under many conditions, they are not well suited to measuring pressure in biological systems. This is because of their relatively large size (2.5 mm diameter would be considered to be a very small strain gauge), the fact that they are electrical and therefore susceptible to malfunction in wet or conductive tissues, their poor resolution and susceptibility to electromagnetic interference.
In-fibre Bragg gratings (FBGs) are optical sensors that have been successfully applied in structural monitoring of various civil, and aeronautical structures. Although they are a proven alternative to conventional electronic sensors, they have not enjoyed widespread use. Several factors contribute to their limited application including cost and a lack of familiarity with optical technologies to users of traditional strain measurement systems.
FBGs have many qualities that are desirable in sensing applications. They are extremely small, with 50 μm outside diameter and sensor gauge lengths as short as a few millimeters. They require only one optical lead for illumination and data acquisition, as opposed to electronic strain gauges which require two leads. They are also unaffected by external electromagnetic fields, making shielding unnecessary. Further to this, because FBGs are non-metallic, they can be used within a magnetic resonance imaging scanner. The mechanical properties of FBGs are similar to those of silica glass, but their small size, especially in relation to their diameter makes them flexible. Therefore they can be bent and twisted to accommodate their host structure. They minimally impact the mechanics of the host structure because of their small size. Despite this, FBGs have not been used extensively in medical and biomedical applications.
One example of a medical application is disclosed in US patent publication number 20020041724. Disclosed is a body compatible fiber optic sensor probe for invasive medical use that includes an optical fiber, a sensing location at which the fiber is configured to provide at least one detectable changeable optical property responsive to a strain within the fiber, and at least one sensing element which undergoes a volumetric change in response to an in body parameter to be sensed. The sensing element is coupled to the fiber in such a way that the volumetric change induces the strain within the fiber so as to vary the detectable changeable optical property. In the preferred embodiment, a FBG is used and a thick polymer coating such as a hydrogel is the sensing element. The polymer coating on the fibre causes the fibre to strain more, when exposed to pressure on its outer cylindrical surface, than if the fibre itself were exposed to the pressure. The pressure is not localized and therefore a relatively large sample volume is needed. Hence, the invention would not be suitable for measuring pressure in confined body spaces, such as intervertebral discs. Further, the use of a polymer that can absorb water could introduce errors in measurements.
It is an object of the present technology to overcome the deficiencies in the prior art.